1. Field of the Invention
The present invention generally relates to radio communications systems and particularly relates to base station apparatuses, mobile station apparatuses, and communications control methods.
2. Description of the Related Art
As a communications scheme to succeed W-CDMA (Wideband Code Division Multiple Access) and HSDPA, Long Term Evolution (LTE) is being studied in a W-CDMA standardization body called 3GPP. Moreover, as radio access schemes, OFDMA (Orthogonal Frequency Division Multiplexing Access) is being considered for downlink, while SC-FDMA (Single-Carrier Frequency Division Multiple Access) is being considered for uplink (see Non-patent document 1, for example).
The OFDMA, which is a scheme for dividing a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective frequency bands to transmit the data, densely arranges the sub-carriers on the frequency axis such that one sub-carrier partially overlaps another sub-carrier without their interfering with each other, making it possible to achieve high-speed transmission and to improve frequency utilization efficiency.
The SC-FDMA is a transmission scheme which divides a frequency bandwidth and transmits using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA, which features a reduced variation in transmission power, makes it possible to achieve wide coverage as well as low power consumption of the terminals.
The LTE is a system for communicating with multiple mobile stations sharing one or more physical channels for both uplink and downlink. In the LTE, the channel shared by multiple mobile stations, which is generally called a shared channel, is a physical uplink shared channel (PUSCH) in uplink and a physical downlink shared channel (PDSCH) in downlink. Moreover, the shared channel, as a logical channel, is an uplink shared channel (UL-SCH) in uplink and a downlink shared channel (DL-SCH) in downlink.
Then, in a communications system using a shared channel as described above, it is necessary to signal for each sub-frame (1 ms in the LTE) which mobile station apparatus the shared channel is allocated to. In the LTE, a control channel used for the signaling is a physical downlink control channel (PDCCH) or a downlink (DL) L1/L2 control channel. Moreover, the physical downlink control channel is also used for reporting acknowledgement information for the uplink shared channel and for a transmission power control command.
Information for the physical downlink control channel includes, for example, a control channel format indicator, downlink scheduling information, acknowledgement information (ACK/NACK), an uplink scheduling grant, etc. (see Non-patent document 2, for example.) The control channel format indicator may be called a physical control format indicator channel (PCFICH).
Moreover, downlink scheduling information includes, for example, information on allocating a downlink resource block for the downlink shared channel, an ID of a UE, the number of streams, information on a pre-coding vector, a data size, a modulation scheme, information on HARQ (hybrid automatic repeat request), etc. Furthermore, the uplink scheduling grant includes, for example, information on allocating an uplink resource block for the uplink shared channel, an ID of a UE, a data size, a modulation scheme, uplink transmission power information, information on a demodulation reference signal in uplink MIMO, etc.
Furthermore, in the LTE, the HARQ is applied in a MAC layer in communications using the above-described shared channel. For example, for downlink, the mobile station apparatus decodes a downlink shared channel and transmits, to the base station apparatus, acknowledgement information based on the decoded result (CRC check result) using an uplink control channel. Then, the base station apparatus performs retransmission control according to the contents of the acknowledgement information. The contents of the acknowledgement information are expressed as ACK (acknowledgement), which indicates that a transmit signal has been received properly or NACK (negative acknowledgement), which indicates that it has not been received properly. In the meantime, for uplink, the base station apparatus decodes the uplink shared channel and transmits, to the mobile station apparatus using the downlink control channel, acknowledgement information based on the decoded result (CRC check result). Then, the mobile station apparatus performs retransmission control according to the contents of the acknowledgement information. The contents of the acknowledgement information are expressed as ACK (acknowledgement), which indicates that a transmit signal has been received properly or NACK (negative acknowledgement), which indicates that it has not been received properly. The downlink control channel may be called a physical HARQ indicator channel (PHICH).
Here, in the downlink HARQ, which is called Asynchronous HARQ, a retransmit timing with respect to an initial transmit timing is not particularly specified. In the meantime, in the uplink HARQ, which is called Synchronous HARQ, a retransmit timing with respect to the initial transmit timing is specified. More specifically, in the uplink retransmission, a periodic transmission is conducted with the initial transmit timing as a start timing. FIGS. 1 and 2 show overviews of the Asynchronous HARQ and the Synchronous HARQ.
In the LTE, for packet data with a somewhat constant transmission speed (e.g., VoIP or streaming), a scheduling scheme for allocating radio resources for each of constant periods rather than a best-effort type scheduling scheme for allocating radio resources to achieve high efficiency is being proposed (see Non-patent document 3, for example). The proposed scheduling scheme is called persistent scheduling or semi-persistent scheduling, for example.
There is shown a method of allocating downlink radio resources when persistent scheduling is applied. As shown in FIG. 3, the base station apparatus transmits a downlink shared channel for each of the constant periods (for each 20 ms as shown). In the persistent scheduling, transmitting and receiving in advance at a known transmit timing between the base station apparatus and the mobile station apparatus makes it possible to reduce downlink (DL) scheduling information for initial transmission and to effectively utilize downlink radio resources. Moreover, it suffices for the mobile station apparatus to conduct downlink reception for each of the constant periods, making it possible to reduce battery power consumption.
Furthermore, as shown in FIG. 4, when some packet data are erroneously received, the packet data are retransmitted by the base station apparatus. Here, downlink retransmission in the LTE is asynchronous. In the persistent scheduling, second and subsequent transmissions are conducted at an arbitrary timing (“retransmit timing” shown), when a HARQ round trip time (RTT) has elapsed since the initial transmit timing. Here, the retransmission is accompanied by the downlink scheduling information. In other words, the mobile station apparatus can receive the downlink scheduling information to receive packet data transmitted in the second and subsequent transmissions.
Non-patent document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006
Non-patent document 2: R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding
Non-patent document 3: R1-060099, Persistent Scheduling for E-UTRA, January, 2006